Relevance - Chronic bacterial infections cause significant morbidity and mortality worldwide. The World Health Organization estimates that 1.7 million people per year die from Mycobacterium tuberculosis infections, which can lie dormant in a healthy host for years. Salmonella enterics serotypes can also cause chronic asymptomatic infections in which an apparently healthy host sheds the pathogen intermittently. This situation sets the stage for acute typhoid fever in approximately 17 million naTve hosts each year. Our plan is to understand how, at the molecular and cellular levels, chronic infections are established and maintained. To accomplish this goal we will use a mouse model of persistent Salmonella enterica infection. This model is an excellent one because it examines a natural infection in a genetically tractable animal. A major focus of our proposed work is the Res (Regulation of capsule synthesis) signaling pathway, which we found contributes to the establishment of persistent infection in our mouse model. Using bacterial mutants and DNA microarrays, we have identified 127 Res-activated genes, including genes that either promote or limit in vivo colonization by Salmonella. This finding is consistent with our hypothesis that the establishment of chronic disease is prevented by both too much and too little bacterial colonization;too little colonization may allow for bacterial clearance with few or no signs of acute infection, whereas too much colonization may stimulate the immune system to efficiently eliminate the bacteria. We will begin by exploring the roles of two critical Res-activated genes that we have shown oppositely affect colonization. We also have data indicating that as-yet-uncharacterized members of the Rcs-regulon enhance Salmonella survival in interferon-g stimulated macrophages, and we will identify these genes. Interferon-g stimulated macrophages are relevant to chronic infection because they, rather than resting macrophages, are likely encountered by Salmonella upon re-seeding. In addition, we will pursue our recent findings that the host interferon-g signaling pathway is important for the creation of a cellular niche in which Salmonella replicates during persistent infection but which has not been observed during acute infection.